PrepTest 149, Section 1, Question 17
Discharges of lightning from a volcanic ash cloud(63 more words not shown)
Which one of the following(7 more words not shown)
- AThe highest point(13 more words not shown)
- BLightning discharges can(11 more words not shown)
- CWeather radar is(21 more words not shown)
- DA volcanic ash(23 more words not shown)
- ELightning discharges are(14 more words not shown)
Reading the argument
Can the lightning really tell us how high the cloud is?
The passage gives us a starting connection: lightning occurs only above five kilometers, and discharges become more frequent as the cloud rises farther. So lightning could act as a substitute measurement where weather radar isn't available.
My first concern is whether something else, such as temperature or pressure, also affects the lightning. If so, we'd need to know how to interpret the discharge data before confidently reading off a height. Another influence wouldn't automatically make the data useless—we might be able to account for it—but it could complicate the proposed measurement.
What we’re looking for
For a required assumption, I'd initially look for something protecting that connection between lightning and altitude. The conclusion only says lightning can sometimes be our sole reliable indicator, so we shouldn't demand that it work perfectly in every situation.
Working the answers
- A
Some ash clouds could stay below five kilometers and never produce lightning. That wouldn't prevent lightning from being useful in other cases. The conclusion's 'sometimes' doesn't require every cloud eventually to rise above the threshold.
- BCorrect
This gets at an earlier practical requirement than my prediction: can we obtain the lightning data where we lack radar? The lightning can indicate altitude only if we can detect it.
If lightning couldn't be detected in any region without weather radar, the argument's proposed alternative wouldn't help in the very places used to justify it. We need at least some overlap between radar being unavailable and lightning being detectable. B supplies that modest requirement; it doesn't promise complete coverage or settle every question about interpreting the data.
- C
How radar performs on ash clouds compared with ordinary clouds doesn't tell us whether lightning is useful where radar is absent. Radar could be somewhat less accurate for ash clouds and still provide a measurement where it exists. The argument doesn't depend on this comparison of radar's accuracy.
- D
Radar can be unavailable because a region lacks the equipment. We don't have to assume that clouds above five kilometers generally extend beyond radar's reach. D proposes a particular reason radar might fail to cover a cloud; that reason isn't required for the argument's stated problem of unavailable radar.
- E
Cloud size could be another influence worth investigating, but the argument doesn't need the blanket rule that large clouds never produce more frequent discharges than small ones. Perhaps a useful estimate could account for size, or perhaps lightning is reliable in some circumstances despite that difference. E is stronger than what the limited conclusion requires.
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